In a cellular communication system, a User Equipment (UE) may camp in a cell in a so-called idle-mode. The UE is associated with the cell, but has no permanent active links established with the radio node of the cell. The UE may thereby save energy buy refraining from frequent communication with the radio node. However, even when camping on a serving cell in idle-mode, an UE must regularly search for a better serving cell. This inevitably involves powering up of circuitry and controllers.
In the current Narrow Band Internet of Things (NB-IoT) solution, no measurement reporting by the UEs nor any handover procedure is defined to support connected mode mobility for UEs between cells. This is due to requirements to keep both the cost of and energy consumption by the UEs at a very low level. The ability of supporting measurement reporting and handover should increase both cost and energy consumption.
When a stationary UE first performs a cell selection, it will select the first suitable cell fulfilling the cell selection criterion in order to keep the initial cell selection delay at minimum. If this serving cell still fulfils the cell selection criterion at the next measurements occasion, the UE may choose not to perform further cell reselection measurements. This means that a stationary UE can camp in a suitable cell that is not necessarily the best available cell for a substantial time.
The re-selection procedure involves scanning and measurement on several frequencies and cells. It is therefore desirable that this procedure is rarely triggered in order to preserve the battery lifetime of UE. Therefore, the system information from serving cell controlling when to do measurements and cell re-selection is intentionally configured to seldom trigger.
On the other hand, to optimize resource usage in the radio network, UEs camping on a cell in a radio condition which require high number of repetitions, i.e. NB-IoT UEs connecting to the cell at a high Coverage Enhancement (CE) level, should be kept to a minimum. It is therefore from a network perspective desirable for such UEs to find a better cell and re-select accordingly, hence the system information should be configured to frequently trigger.
The configuration of the system information is hence conflicting objectives for the network and the UE. Setting the thresholds too low will result in that few, or no, UEs will ever make measurements for cell re-selection and misses opportunities of camping on cells with better receiver levels. Setting the thresholds too high will result in high number of repetitive measurements and attempts for cell re-selection which drains UE battery.